Climatological Background:
The climate of central and eastern Argentina varies from semi-arid in the mountainous west to humid subtropical across the vast, flat grasslands (pampas) of the east.  Arid conditions prevail across the southern part of Argentina where the Patagonian Desert is located.  Precipitation generally occurs throughout the year across central and eastern areas but displays a marked seasonality, with low rainfall from May to September, increasing precipitation in October, and a maximum during austral summer.  In the more arid regions of northern Argentina, east of the Andies, the period May to September is generally dry with the onset of the rainy season occurring in October.
 

Flood Risk, Streamflow and ENSO
Given the shallow slope of the terrain across the vast grasslands of the pampas east of the Andes, runoff is limited and excessive rainfall often leads to widespread soil saturation and flooding.  Several flooding episodes have occurred over the past 10 years including events in 1998, 1994 and 1993.  The floods in 1998 were severe (the city of Parana observed 1,075 mm of rainfall during May 1998; annual average is 994 mm) with many areas observing record amounts of rainfall.  Flooding and periods of unusually high streamflow are often associated with El Nino events, with flooding often occurring towards the end of the rainy season between March and May.  Crop yields in the fertile pampas have also been linked to the phase of ENSO.  Flooding is not always associated with El Nino with flood events also occurring during both La Nina events and in non-ENSO years.  See figure below.
 
 

Nino 3.4 Sea Surface Temperature Index

"Pampas" Rainfall Anomaly (34-38S, 58-65W)

 
 
 

Recent Rainfall Anomalies:
The average rainfall over the past 12 months has been well above the long term average across much of central and eastern Argentina including the pamapas (the ENSO state over this period  was that of a fading La Nina).  Heavy rainfall in September 2001 marked an unusually early onset of the rainy season across sections of the pampas.  Precipitation continued to be well above average in the following month of October with some areas receiving greater than 300 mm, more than twice the long-term average for the month.  The IRI 3 month WASP (weighted anomaly standardized precipitation) index is shown below with areas shaded in dark green indicating extremely wet conditions.
 
 

Map courtesy of the University of Texas Library

WASP is an acronym for the Weighted Anomaly Standardized  Precipitation index which is based on monthly rainfall departures from the long-term (30-year) average. ------------------------- Standardized Precipitation: To be able to compare regions with varying amounts of climatological  precipitation, monthly rainfall departures have been divided by their  standard deviation or standardized. Weighted Anomaly: To avoid an exaggerated influence of standardized precipitation values observed during months which are usually dry, they are weighted by the  fraction of annual rainfall which usually occurs during a given month.

Recent Impacts on Agriculture
The recent heavy rainfall and flooding, though not nearly as severe as that seen in previous years, has damaged roads and property and impacted various sectors including agriculture in the Pampas.   Immediate concerns in the agricultural sector include the possible spread of disease among wheat crops grown in the region due to the unusually wet conditions.  In addition, planting season for corn in the Pampas generally starts in October (see figure below) and the saturated soils stifle field work and inhibit proper seed germination.  This is situation is especially acute in important growing areas in Northeast Buenos Aires province as well as the southern regions of Cordoba and Sante Fe provinces.  Overall, the US Department of Agriculture (USDA) anticipates acreage in corn production to decline more than 10% (other estimates are as high as 30%) from the previous year, which itself was below the long term average due to wet conditions.  Given the current situation, many growers are thinking of shifting some of their corn production to soybeans.  In addition to current market conditions favoring soybean production, soybeans in general tend to have a higher return per input cost of raising the crop according to the USDA report^*.
 

IRI Seasonal Climate Forecast
Oceanic conditions, a crucial factor in producing seasonal climate forecasts, are currently weak including a near-neutral state of ENSO.  This, combined with uncertainty in the future state of the oceans for the coming season, is reflected in the precipitation probabilities shown in the IRI seasonal forecast for South America below.  The IRI seasonal forecasts are given in terms of "terciles", that is, the probability that rainfall for the season will be above average (top tercile), near-average (middle tercile) or below-average (bottom tercile).  For the period December 2001 - February 2002, the current forecast indicates a slightly enhanced probability of below-average precipitation across sections of northeastern Argentina.  These forecasts are produced monthly at the IRI and are available at:  http://iri.columbia.edu/climate/forecast/net_asmt/
 

^*Argentina Grain and Feed Monthly Update for November Lockup 2001, USDA GAIN Report #AR1071, November 1, 2001.
 

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